There are several modes proposed as the display mode for a liquid crystal display device. As one of the most general mode, is a twisted nematic mode (hereinafter referred to as TNM). This TNM is available for use in a calculator, watch, large-sized display and so on. However, recently there are increasing demands being made of the display device which needs high duty driving. This is the reason that there is the limit of the high duty driving in the above TNM, thereby not obtaining the sufficient contrast ratio. Accordingly, attention is being given to the method of using the birefringence mode again as shown in Japan-Laid Open No. 60-50511 and the others.
In the Japan-Laid Open No. 60-50511, the liquid crystal display device is disclosed, wherein the twisted angle of the liquid crystal molecule is large, between 160.degree. and 200.degree., a pair of polarizers are provided on the upper and lower liquid crystal cells, and the angle between the above polarizing axis (absorption axis) and the molecular axis direction of the liquid crsytal molecule adjacent to the electrode substrate falls within the range from 30.degree. to 60.degree.. In case of the liquid crystal display device using the above birefringence mode, it results in the background and the display portion of the display screen being colored by the dispersal due to the wavelength of the light. For example, in case of the positive display, the background is displayed as a green to reddish yellow color and the display portion is displayed as a blue. On the other hand, in case of the negative display, the background is displayed as a dark blue and the display portion is displayed as a light yellow color. As the display device, the color shade of green to reddish yellow is not desirable because it is difficult for men to accept mentally.
With respect to the above display, some experiments have been conducted on the background color as reported in "Investigation Report for VDT Guide Line", Japan Electronic Industry Development Association, March of 1985. From this article, it has been considered that the light color is desiable for the background color in case of the positive display device.
However, as mentioned above, the background color is never displayed as a light color in case of where the birefringence mode is employed. FIG. 3 shows the background color on the xy chromaticity diagram, wherein the optical anisotropy of the liquid crystal is n=0.12, the thickness of the liquid crystal layer is 8.mu., the twist angle of the liquid crsytal molecle is 180.degree., and the angle between the nearest optical axis of the liquid crystal molecule and the polarizing axis of the polarizer is 45.degree.. In particular, the background color inpropotion to the thickness of the liquid crystal layer from 6.5.mu. to 8.mu.. According to FIG. 3, it is clear that the background color is not a pale color even though the thickness of the liquid crystal layer is varied.
Next, in case of the negative display device, the light source is provided at the back side of the liquid crystal display portion in the liquid crystal display device employing the above birefringence mode, then viewer observes the liquid crystal display portion. So, the background color of the liquid crystal display portion becomes blue. Background color varies and is sensitive depending on the thickness of the liquid crystal layer. Especially, since the blue color is the area where it is sensed that the color difference is largest, it tends to occur the color shading in the background in case of using the bright light source. Therefore, it is very difficult for us to obtain the uniform display surface of the background color.
The present invention is to solve the above defects of the liquid crystal display device employing the conventional birefringence mode as mentioned above. The object of the present invention is to provide an improved display device in the liquid crystal display device by the birefringence mode, wherein in case of the positive display, the background color is displayed as a pale color, thereby making the display device having the high contrast ratio, on the other hand, in case of the negative display, the background color is without a color shading.